Atoms, Elements, and the Periodic Table
Core concepts
An atom has a nucleus of protons and neutrons surrounded by electrons. Atomic number (protons) defines the element; mass number is protons plus neutrons. Isotopes share a proton count but differ in neutrons. Electrons occupy energy levels; the outermost (valence) electrons govern bonding. The periodic table arranges elements by increasing atomic number, with groups (columns) sharing valence-electron patterns and periods (rows) showing repeating trends. Metals are generally on the left, nonmetals on the right, and metalloids along the staircase. Atomic radius decreases across a period and increases down a group; ionization energy generally increases across a period.
Traps to watch
- Confusing atomic number with mass number, or protons with neutrons.
- Assuming the atomic mass listed is a whole number; it is a weighted average of isotopes.
- Thinking an ion has changed its proton count; ions differ only in electrons.
- Mixing up groups and periods.
- Believing electrons are in fixed circular orbits rather than probability regions.
Worked example
An atom has 11 protons, 12 neutrons, and 11 electrons. Atomic number = 11 (sodium). Mass number = 11 + 12 = 23. Because it has one valence electron in group 1, it tends to lose one electron, forming a +1 ion with 10 electrons.
Rule of thumb
Protons identify the element; neutrons change the mass; electrons change the charge. Valence electrons predict reactivity.